1. Field of the Invention
The invention is generally related to structures used offshore in the drilling and production of hydrocarbons and more particularly to floating caissons used in such operations.
2. General Background
In the offshore drilling industry, there are many operational issues and difficulties that must be addressed. A conventional subsea BOP (blow-out-preventer) stack riser is large and heavy and normally requires syntactic foam for additional buoyancy. This results in an overall diameter of forty-two inches, which presents a relatively large area that is readily affected by current loads, causing a substantial lateral offset between the surface and the seafloor. Drilling risers are normally supported on hydraulic tensioners with pneumatic accumulators to provide a relatively constant tension variation with stroke. These tensioners are expensive and limited in capacity. Further, because they are mechanical and use wire rope, one hundred percent redundancy is needed. Offshore drilling operations from a floating vessel are normally carried out with a subsea BOP stack in conjunction with a riser that carries the mud returns back to the surface. Alternatively, the pressure risers have been used with the BOP stack on the surface and no shut off mechanism at the seafloor. The first configuration locates complicated and expensive equipment at the seafloor while the second configuration has the disadvantage of no shut off mechanism at the seafloor.